Desloratadine (Clarinex) Side Effects: What to Expect and When to Call Your Doctor

Desloratadine (Clarinex) is generally well tolerated — in clinical trials, the rate of adverse events was similar between Desloratadine and placebo. That said, like any medication, it can cause side effects in some people. Here's what you need to know before you start taking it, and what symptoms should prompt a call to your doctor.

Does Desloratadine Cause Drowsiness?

Desloratadine is a second-generation antihistamine specifically designed to minimize sedation. Unlike older antihistamines like diphenhydramine (Benadryl), Desloratadine does not readily cross the blood-brain barrier. In clinical trials, it was not associated with significantly higher drowsiness than placebo at the recommended 5 mg dose. However, a small number of patients may notice mild fatigue or drowsiness, especially when first starting the medication. If you experience notable drowsiness, take Desloratadine in the evening instead of the morning.

Common Side Effects of Desloratadine (Clarinex)

The following side effects were reported in clinical trials at rates greater than 2% and more frequently than placebo:

Headache (14% Desloratadine vs. 13% placebo)

Nausea (5% vs. 2%)

Fatigue (5% vs. 1%)

Dizziness (4% vs. 3%)

Pharyngitis (sore throat) (3% vs. 2%)

Dyspepsia (indigestion) (3% vs. 1%)

Myalgia (muscle pain) (3% vs. 1%)

Dry mouth (reported during post-marketing)

Notice that the differences between Desloratadine and placebo are small for most of these effects. Many of these symptoms may simply be from the allergy condition itself, not the medication.

Serious Side Effects: Rare but Important to Know

Serious side effects from Desloratadine are rare. Post-marketing surveillance has identified the following, though they are uncommon:

Hypersensitivity reactions: This is the most important serious side effect. Symptoms include rash, itching, hives (urticaria), swelling (especially of the face, lips, tongue, or throat), difficulty breathing, and anaphylaxis. If you experience any of these, stop Desloratadine immediately and seek emergency care.

Tachycardia (rapid heartbeat): Reported rarely in post-marketing experience. Contact your doctor if you notice an unusually rapid or irregular heartbeat.

Elevated liver enzymes / hepatitis: Very rare. If you develop yellowing of the skin or eyes (jaundice), dark urine, or severe abdominal pain, contact your doctor promptly.

Psychomotor hyperactivity, movement disorders: Rare cases of movement disorders (including dystonia, tics, extrapyramidal symptoms) and seizures have been reported in post-marketing experience.

Does Desloratadine Affect the Heart (QT Interval)?

No clinically significant QTc prolongation has been shown with Desloratadine at recommended doses, even when co-administered with drugs that increase Desloratadine levels (like Ketoconazole or Erythromycin). This is an important advantage over some older antihistamines that can affect cardiac conduction.

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Special Populations: What to Watch For

Kidney or liver disease: Dosing adjustment (every 48 hours instead of daily) is recommended. Tell your doctor if you have kidney or liver problems before starting Desloratadine.

Pregnancy: Data on Desloratadine use in pregnancy is limited. Use only if clearly necessary during pregnancy. Inform your doctor.

Breastfeeding: Desloratadine passes into breast milk. Discuss with your doctor whether to stop nursing or stop the medication.

Phenylketonuria (PKU): The orally disintegrating tablet formulation may contain phenylalanine (aspartame). Patients with PKU should alert their doctor.

When Should You Call Your Doctor?

You develop a skin rash, hives, or swelling after starting Desloratadine

You notice an unusually fast or irregular heartbeat

You develop jaundice, dark urine, or severe nausea/abdominal pain

Side effects like headache or nausea are severe or don't improve after the first week

You're pregnant or breastfeeding and need guidance on whether to continue

Seek emergency care if you experience difficulty breathing, severe swelling of the face or throat, or signs of anaphylaxis after taking Desloratadine. Call 911 or go to the nearest emergency room.

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The Bottom Line on Desloratadine Side Effects

Desloratadine is one of the better-tolerated allergy medications available. Serious side effects are rare. Most people take it daily without notable problems. If you're curious about how it interacts with other drugs, see our guide on Desloratadine drug interactions. And for a complete overview of the medication, read What Is Desloratadine? Uses, Dosage, and What You Need to Know.